1. Field of the Invention
The invention relates to an abnormality detection apparatus and an abnormality detection method for a multi-cylinder internal combustion engine, and more particularly to an apparatus and a method for detecting a relatively large variation in air-fuel ratio between cylinders in a multi-cylinder internal combustion engine.
2. Description of Related Art
In general, in an internal combustion engine equipped with an exhaust gas control system that utilizes a catalyst, in order to perform purification of a pollutant in exhaust gas by a catalyst at high efficiency, it is essential to control a mixing ratio between air and fuel of an air-fuel mixture burned in an internal combustion engine, i.e., an air-fuel ratio. In order to control the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine and feedback control is performed such that the air-fuel ratio detected by the air-fuel ratio sensor is caused to match with a predetermined target air-fuel ratio.
On the other hand, in a multi-cylinder internal combustion engine, air-fuel ratio control is usually performed on all cylinders by using the same control amount. Therefore, even when the air-fuel ratio control is executed, there are cases where the actual air-fuel ratio varies between the cylinders. At this point, when the degree of the variation is small, the variation can be compensated by air-fuel ratio feedback control, and the pollutant in exhaust gas can be purified by the catalyst so that the variation does not affect exhaust emission and does not present a problem.
However, for example, when a fuel injection system of a part of the cylinders fails and the variation in air-fuel ratio between the cylinders is thereby increased, the variation deteriorates the exhaust emission and presents a problem. The large variation in air-fuel ratio that deteriorates the exhaust emission is desirably detected as an abnormality. In particular, in the case of a vehicle internal combustion engine, in order to prevent the running of a vehicle with deteriorated exhaust emission beforehand, it is required to detect the abnormal variation in air-fuel ratio between the cylinders in an on-board state (so-called OBD; On-Board Diagnostics).
For example, in an apparatus described in Japanese Patent Application Publication No. 2010-112244 (JP-2010-112244 A), when it is determined that an abnormal air-fuel ratio occurs in any of cylinders, an injection time period, during which fuel is injected to each cylinder, is reduced by a predetermined time period until a misfire occurs in the cylinder with the abnormal air-fuel ratio, and the abnormal cylinder is thereby identified.
In the case where the abnormal air-fuel ratio occurs in any of cylinders, when the fuel injection quantity of the cylinder is forcibly changed (increased or reduced), the rotational variation relating to the cylinder is significantly increased. Consequently, by detecting the increase in rotational variation, it is possible to detect the abnormality of the internal combustion engine, particularly the abnormal variation in air-fuel ratio between the cylinders of the internal combustion engine. Specifically, the fuel injection quantity of a predetermined target cylinder is changed and, based on the rotational variations relating to the target cylinder detected before and after the changing, it is possible to detect the abnormal variation in air-fuel ratio between the cylinders.
However, when the fuel injection quantity is changed, there is a case where the operation condition of the internal combustion engine is changed from that before the change. Therefore, in this case, values of the rotational variations detected before and after the change are values detected under different operation conditions so that abnormality detection based on the values may not be performed with sufficient accuracy.